Method to prevent low temperature degradation of zirconia

ABSTRACT

The invention is directed to a method of producing the material that is unaffected by the low-temperature degradation, humidity-enhanced phase transformation typical of yttria-stabilized zirconia, as well as of yttria-stabilized tetragonal zirconia polycrystalline ceramic (Y-TZP). Because of the high fracture toughness and high mechanical strength, this class of materials is widely used, including as implants, such as for the packaging material for small implantable neural-muscular sensors and stimulators. The destructive phase transformation rate is dramatically reduced by coating the surface of the Y-TZP component with dense alumina by a physical vapor deposition process, preferably ion beam assisted deposition.

CROSS REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.10/711,154, filed Aug. 27, 2004.

FIELD OF THE INVENTION

This invention relates to a coating material and a method of increasingthe useful life of an yttria-stabilized zirconia structure whenimplanted in living tissue.

BACKGROUND OF THE INVENTION

One widely employed bioceramic is alumina, which is considered bioinert.The search for an ideal bioceramic has included alumina, hydroxyapatite,calcium phosphate, and other ceramics. The first use of aluminas forimplants in orthopedics and dentistry was in the 1960's. They were lateremployed in hip prostheses as early as 1970. Since those early daysthe.quality and performance of aluminas have improved. High-purity,high-density, fine-grained aluminas are currently used for a wide rangeof medical applications, e.g. dental implants, middle ear implants, andhip or knee prostheses.

Although the aluminas currently available perform satisfactorily, afurther improvement in strength and toughness would increase the safetyfactor and may extend usage to higher stressed components. A proposedcandidate to add to this list is stabilized-zirconia, because of itspotential advantages over alumina of a lower Young's modulus, higherstrength, and higher fracture toughness. Another advantage ofstabilized-zirconia is low-wear residue and low coefficient of friction.Because, zirconia undergoes a destructive phase change at between 1000°and 1100° C., changing from monoclinic to tetragonal, phasestabilization admixtures of calcia, magnesia, ceria, yttria, or the likeare required.

Tetragonal zirconia polycrystalline ceramic, commonly known as Y-TZP,which typically contains 3 mole percent yttria, coupled with a smallgrain size, results in the metastable tetragonal state at roomtemperature. Under the action of a stress field in the vicinity of acrack, the metastable particles transform, accompanied by a 3% to 4%volume increase, by a shear-type reaction, to the monoclinic phase.Crack propagation is retarded by the transforming particles at the cracktip and by the compressive back stress on the crack walls behind thetip, due to volume expansion associated with transformation to themonoclinic phase.

The well-known transformation toughening mechanism is operative inzirconia ceramics whose composition and production are optimized suchthat most of the grains have the tetragonal crystal structure. TheseY-TZP ceramics, most notably their mechanical properties in air at roomtemperature, are superior to those of zirconia-toughened aluminas and toother classes of zirconias. While the biocompatibility of Y-TZP ceramichas not been fully assessed, it has been preliminarily investigated.

For example, in one study by Thompson and Rawlings [see I. Thompson andR. D. Rawlings, “Mechanical Behavior of Zirconia and Zirconia-ToughenedAlumina in a Simulated Body Environment,” Biomaterials, 11 [7] 505-08(1990)]. The result was that Y-TZP demonstrated a significant strengthdecrement when aged for long periods in Ringer's solution and wastherefore unsuitable as implant material.

Drummond [see J. L. Drummond, J. Amer. Ceram. Soc., 72 [4] 675-76(1989)] reported that yttria-stabilized zirconia demonstratedlow-temperature degradation at 37° C. with a significant decrement instrength in as short a period as 140 to 302 days in deionized water,saline, or Ringers solution. He also reports on similar observation byothers, where yttria-stabilized zirconia demonstrated a strengthdecrement in water vapor, room temperature water, Ringers solution, hotwater, boiling water, and post-in vivo aging.

Y-TZP components suffer a decrement in strength properties afterexposure for only a few days to humid environments. This degradation ofmechanical properties occurs when moisture is present in any form, forexample, as humidity or as a soaking solution for the Y-TZP component.Y-TZP components have been observed to spontaneously fall apart aftertimes as short as a few weeks in room temperature water. This is ofparticular importance in living-tissue implanted devices that containcomponents made of this class of material. Long- term implantation ofdevices that contain yttria-stabilized (or partially-stabilized)zirconia components is not feasible with available materials.

One approach to preventing the low-temperature degradation of zirconiathat was doped with 3 mole percent yttria is presented by Chung, et al.[see T. Chung, H. Song, G. Kim, and D. Kim, “Microstructure and PhaseStability of Yttria-Doped Tetragonal Zirconia Polycrystals Heat Treatedin Nitrogen Atmosphere,” J. Am. Ceram. Soc., 80 [10] 2607-12 (1997).].The Y-TZP sintered material was held for 2 hours at 1600° or 1700° C. inflowing nitrogen gas.

Another approach to preventing low temperature degradation of zirconiain biomedical implants is disclosed by Lasater in U.S. application Ser.No. 10/853,922, while Jiang, et al., U.S. patent application Ser. No.10/629,291, disclose a method of overcoming the pest low-temperaturedegradation in yttria-stabilized zirconia.

Analysis showed that the resulting surface consisted of cubic grainswith tetragonal precipitates, while the interior was only slightlyaffected by the nitrogen exposure. Chung reported that low-temperaturedegradation was prevented because degradation of Y-TZP started at thesurface, which is protected from degradation by the stable cubic phase.

Koh, et. al investigated an encapsulating layer deposited on the surfaceof tetragonal zirconia polycrystals to prevent the low-temperaturedegradation of zirconia that was doped with 3 mole percent yttria [seeYoung-Hag Koh, Young - Min Kong, Sona Kim and Hyoun-Ee Kim, “ImprovedLow-temperature Environmental Degradation of Yttria-Stabilizedtetragonal Zirconia Polycrystals by Surface Encapsulation”, J. Am.Ceram. Soc, 82 [6] 1456-58 (1999)]. The layer, composed of silica andzircon, was formed on the surface by exposing the zirconia specimensnext to a bed of silicon carbide powder in a flowing hydrogen atmospherethat contained about 0.1% water vapor at 1450° C.

An alternate material and an easy to apply method of producing stablematerial to prevent the detrimental low-temperature phase change areneeded.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 presents a schematic representation of an ion beam assisteddeposition apparatus.

FIG. 2 presents a chart of the effect of the alumina coating onmonoclinic phase increase with aging time.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A broadly applicable material and method of producing the materialbegins with the densified as-sintered, tetragonal zirconiapolycrystalline ceramic (Y- TZP) material that is produced by processesthat are known to those skilled in the art, containing about 3 molepercent of yttria.

The Y-TZP material is formed into a desired final shape and is thencoated with alumina by the ion beam assisted deposition (IBAD) processof FIG. 1. The IBAD process creates a conformal coating, versus aline-of-sight coating, of alumina. The resulting alumina coating isdense and strongly adherent to the Y- TZP substrate 4. Unexpectedly,this coating has been shown to prevent or minimize the destructivelow-temperature degradation of Y-TZP ceramic that occurs in moistenvironments. Alternate deposition methods are known, includingmagnetron sputter deposition and ion implantation coating deposition,however neither has been investigated for alumina on Y-TZP.

In a preferred embodiment, the coating thickness is at least about 1.6microns. If the coating is less than about 1.6 microns thick then it ismore likely to allow low temperature degradation of the Y-TZP ceramic,while if the coating thickness is greater than about 10 microns, thenthe coating is more likely to crack or spall off of the substrate. Theaverage grain size of the alumina is preferably less than about 0.5micron average, as measured by the line intersection method. Thisincreases the toughness of the coating.

The IBAD process apparatus 2, FIG. 1, involves placing a substrate 4,which is also often referred to as the “target”, to be coated on asubstrate holder 6. The substrate is heated to about 300° C. Thesubstrate holder 6 preferably rotates slowly at about one revolution perminute, to assist in obtaining a uniformly thick and dense coating onsubstrate 4. An ion gun 8, substrate holder 6, and e-beam evaporator 12are located near the substrate in an environmentally controlled chamber,which is preferably a vacuum chamber that allows an inert gas,preferably argon, to be backfilled into the chamber with a small amountof oxygen. In alternate embodiments, other inert gases, such asnitrogen, or mixtures of inert gases may be utilized in combination withoxygen. In a preferred embodiment, there are two sources of argon; oneto the ion gun and one to the IBAD chamber.

The ion gun 8 includes a source of the desired coating, preferably analumina source 16, in a preferred embodiment. An ion beam 10 isgenerated wherein the energetic ions of alumina are directed toward thesubstrate 4. Simultaneously and continuously with the release of theions, the e-beam evaporator 12 bombards the substrate 4 and the aluminacoating, as it is forming, with an electron beam 14 that is emitted by aheated tungsten filament. It is preferred that the alumina coating becomprised of alpha-alumina or amorphous alumina. Because alpha-aluminais stronger, harder, and has a higher specific gravity than otheraluminas, including amorphous alumina, alpha-alumina is a preferredphase. Amorphous alumina may be converted to alpha-alumina by annealingat about 1000° C. The IBAD process yields both amorphous alumina andalpha alumina in proportions that are dictated by the depositionparameters. A blend of alpha-alumina and amorphous alumina results undercertain deposition parameters. It is believed that rapid quenching ofthe vapor phase results in a predominance of amorphous alumina.Therefore, control of the deposition parameters allows the preferredalpha-alumina phase to be formed in the coating on substrate 4.

It is known to those skilled in the art that the resulting coating has ahigh bulk density, comprising very low open or closed porosity,preferably less than 1.0% total porosity. Therefore, the alumina coatingoffers excellent resistance to moisture penetration, thereby eliminatingor dramatically reducing moisture penetration and diffusion to thesubstrate 4.

EXAMPLE

The base vacuum level is about 1×10⁻⁷ Torr and the working pressure ofargon plus oxygen is about 3×10⁻⁴ Torr. In a chamber of approximatelyone gallon in volume, the flow rates to the ion gun 8 of theargon-oxygen mixture about 10 scc/m argon plus 5.5 scc/m oxygen. Theflow rates to the IBAD chamber are about 5.5 scc/m oxygen and about 3.5scc/m of argon.

The substrate temperature is about 300° C. The electron beam evaporationsource is a solid, dense block of single crystal sapphire alumina with apurity of at least about 99.99 atomic percent.

The deposition rate is about 1.5 angstroms per second at an ion beambombardment energy of about 1000 eV and an ion beam current of about 26mA. In alternate embodiments, the film is bombarded with ions from anion gun with energies typically in the range of 1.0 to 1.5 Kev. As aresult, energy is transferred to the coating atoms, allowing them tomigrate on the surface, and the coating can grow in a more uniformmanner.

A 1.6 micron thick alumina coating was applied by IBAD on a sealedceramic case comprised of Y-TZP. X-ray diffraction analysis wasperformed on this unit prior to and after soaking in 127° C. steam for20, 85, 137, and 201 hours. The X-rays penetrate the thin alumina layerand allow peak detection of 2 Theta angles of 28.2, 30.2 and 31.3degrees. The monoclinic phase fraction is calculated by the modifiedGarvie-Nicholson equation. As presented in FIG. 2, the monoclinic phasepercentage changes with the increase in soak time. Initially the phasetransformation rate of the ceramic coated with 1.6 microns alumina ismuch slower than that of a ceramic having no alumina coating. After 150hours, the monoclinic phase increased abruptly. The alumina coatedceramic self-destructed after 201 hours of soaking, when the monoclinicphase was 49%, which is less than the 70% monoclinic saturation level.

FIG. 2 presents the destructive phase conversion of tetragonal tomonoclinic phase when Y-TZP ceramic that has been left in the as-formedcondition (i.e., uncoated) and Y-TZP ceramic that has been coated by theIBAD process with a thin alumina film are exposed to 127° C. steam in astatic, unstressed state. This accelerated life test, which equates 100hours of soak time with an implant service life of 5.84 years, isdisclosed by Jiang, et al. in U.S. patent application Ser. No.10/651,462.

Obviously, many modifications and variations of the present inventionare possible in light of the above teachings. It is therefore to beunderstood that, within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described.

1. A method of manufacturing a degradation resistant component for usein living tissue, comprising the steps of: providing a componentcomprised of yttria-stabilized tetragonal zirconia polycrystal ceramic;depositing a coating of alumina by ion beam assisted deposition on thecomponent; and selecting deposition parameters to achieve a coatingcomprised of less than about 1.0 percent total porosity.
 2. The methodaccording to claim 1, wherein said depositing a coating comprises thestep of depositing alpha-alumina, amorphous alumina, or a blend thereof.3. The method according to claim 1, wherein said depositing a coatingcomprises the step of depositing a coating comprised of an average grainsize less than about 0.5 microns.
 4. The method according to claim 1,wherein said depositing a coating of alumina comprises depositing acoating comprising a thickness that is greater than about 1.6 micron andless than about 10 microns.
 5. The method according to claim 1, whereinsaid providing a component comprises providing a component comprisingabout 3 mole percent yttria.
 6. The method according to claim 1, whereinsaid selecting deposition parameters comprises selecting an ion beambombardment energy of about 1000 eV and an ion beam current of about 26mA.
 7. The method according to claim 1, wherein said depositing acoating comprises depositing at a rate of about 1.5 angstroms persecond.